Pneumatic tires are often provided with a circumferential tread of a cap/base configuration comprised of an outer ground-contacting tread cap rubber layer with a tread running surface and an internal rubber layer underlying said tread cap rubber layer such as, for example, a tread base rubber layer or intermediate rubber layer between the outer tread cap rubber layer and tread base rubber layer. Such tire constructions are well known to those having skill in such art.
The outer tread cap rubber layer is typically comprised of a rubber composition which provides resistance to tread wear during tire service, stiffness for tire handling purposes and traction for the road surface. Such typically desirable properties are well known to those having skill in such art.
The internal tread underlying rubber layer is typically comprised of a rubber composition which is less stiff than the outer tread cap rubber layer and less hysteretic to promote a reduced internal heat buildup during tire service to thereby provide a transition zone between the outer tread cap rubber layer and the tire carcass. Such typically desirable properties for a tire underlying tread base or intermediate rubber layer are well known to those having skill in such art.
However, some tires are intended to be provided for use as high performance tires which are expected to provide a challenge of enhanced (improved) handling and cornering ability. For this invention it is desired to increase a tire component's stiffness while substantially maintaining its hysteresis (e.g. tire component as a tread's internal underlying tread rubber layer or as a sidewall's apex). For the tire tread, it is desired to promote its stiffness to promote tire handling yet substantially maintain, without significantly increasing, the rubber's hysteresis property, or tendency of internal heat buildup during the tire's service.
While it is well known that a rubber composition's stiffness can normally be increased by increasing its reinforcing filler content, for example by increasing its rubber reinforcing carbon black content, it is also known that such methodology of increasing rubber stiffness through reinforcing filler increase can also be expected to significantly increase the rubber composition's hysteresis and thereby increase its internal heat buildup during tire service with a resultant usually unwanted extent of temperature increase.
It is therefore desired to evaluate significantly reducing the rubber reinforcing carbon black content below an otherwise desirable rubber reinforcement level to thereby promote a reduction of a rubber composition's hysteresis and attendant internal heat buildup combined with promoting stiffness of the tire tread base rubber layer by other means.
For such purpose, it is desired to evaluate use of a syndiotactic-1,2-polybutadiene for a rubber composition for a tire component (e.g. tread base rubber layer or apex) to promote stiffness for the rubber composition while significantly reducing its rubber reinforcing carbon black content to promote a reduction of the rubber composition's hysteresis.
It is recognized that syndiotactic polybutadiene has been used in various rubber compositions for various tire components such as for example, tire carcass and innerliner components. For example, see U.S. Pat. Nos. 5,307,850 and 6,956,093.
It is believed that use of syndiotactic polybutadiene for a tread base rubber composition combined with a significant reduction of rubber reinforcing carbon black content is novel and a departure from past practice and that such combination is warranted for evaluation.
In the description of this invention, the term “phr” where used means “parts of material by weight per 100 parts by weight of rubber”. The terms “rubber” and “elastomer” may be used interchangeably unless otherwise indicated. The terms “rubber composition” and “compound” may be used interchangeably unless otherwise indicated.
A volume electrical resistivity, sometimes referred to herein as electrical resistivity, may suitably be determined by DIN 53682 or ASTM Method D257-92 and reported herein as ohm-cm.